In order to be used as a power device, a SiC MOSFET does not only need to be highly resistant against pressure, but is also desirably a normally-off type transistor in many applications. Even when the SiC MOSFET is a normally-off type transistor, if the threshold voltage Vth is set to near 0 [V], erroneous turn-on occurs due to a change in the gate potential Vg. Therefore, it is necessary to increase the threshold voltage Vth to a certain extent.
In addition, a power device, particularly, a (lateral) DMOSFET having a channel in an in-plane direction needs to realize a low on-resistance (the improvement of the channel mobility of MOSFET).
As a technique to increase the channel mobility of MOSFET, there are a technique to decrease the large interface state density at the SiC/Si oxide film interface so as to reform the SiC/Si oxide film interface (PTL 1) and a technique to implant a channel into SiC from the SiC/Si oxide film interface having a large interface state density (PTL 2).
Specifically, PTL 1 described in [0004] and [0005] that, when a nitriding treatment is carried out on a SiC/Si oxide film (thermal oxidation, LP-CVD) interface right below a gate oxide film in a SiC-n channel MOSFET (nMOSFET), the interface state density decreases so that it is possible to increase the channel mobility.
PTL 2 describes that a Double-Implanted MOSFET (DiMOSFET) in which a short-circuited channel (implanted channel) obtained by counter-doping nitrogen into a channel is used can improve the channel mobility. Furthermore, PTL 2 describes that, in a case in which the threshold voltage is preferentially increased, the dose amount of nitrogen being counter-doped decreases.